1. Field of the Invention
The present invention relates to the image forming method and apparatus wherein the control of the concentration of the output of image forming apparatus particularly laser beam printer and the like can be executed with stability.
2. Related Background Art
In recent years, non-impact printers are drawing attention because of their advantages, such as high print quality with low noise, over impact printers. Among other types of non-impact printers, laser beam printers, which utilize electronic photography and photography techniques, have become popular because such printers record with high precision. The semiconductor laser used as the light source in such printers can have its gradation easily varied by pulse width modulation (PWM), whereby high quality halftone images can be formed.
However, when high speed modulation is conducted, the semiconductor laser is subject to output fluctuations caused by delayed rising of laser output or temperature, and if short (quick) pulse modulation is conducted, the laser output lacks stability. Further in the gradient expression by PWM method, when pulse interval is short, beam shape generally has Gaussian distribution. Consequently, as shown at "a" in FIG. 2, when the laser power receives substantial brightness modulation, fluctuation of laser output produces variation of concentration, making the image reproduction unstable.
With the plate exposure apparatus for offset printing, it is necessary to do binary control such as control of presence or non-presence of resist layer and if short pulse interval as stated above exists, binarization is difficult and it is difficult to obtain an appropriate binary image.
Furthermore, reduction of the beam spot diameter which is presently 20 to 30.mu. presents serious problems such as generation of nonuniformity of beam shape caused by aberration, shallower depth of focus which necessitates stricter accuracy of apparatus, more complicated optical system, etc.
Another frequently used method as the method to reproduce gradation of darkness of the image is the method known as the "dither method" and "concentration pattern method". Generally speaking, the dither method is used when the image of high resolution such as those of high resolution printer or copier are handled while the concentration pattern method is used more frequently when input pixel number is relatively few as in the case of recording of TV input image.
According to dither method or concentration pattern method, when the concentration of input image signal is constant for a wide range, dot pattern which is the aggregate of blackened display pixels repeats the same pattern. In regard to the resolution, dither method is superior but in regard to gradation, both are equivalent. One of the important factors for gradation is the composition of threshold matrix which is roughly classified into the following two kinds.
(1) Starting with the center of the display area as the nucleus, pixels turn black one after another to form the dot (dot concentration type).
(2) Dot is formed by the pixels blackened one after another in such a way that the spatial frequency of dot becomes as high as possible in the display area (dot dispersion type).
When image quality is evaluated by using these two kinds of matrix, if the display pixels are arranged at high density in matrix form in the display area corresponding to one pixel of image signal, at the blackening of small number (particularly one) of display pixel in low concentration time, protrusion of blackening to the surrounding area becomes non-negligible because individual display pixel is small. Therefore, in the case of dot dispersion type, there is the drawback that the blackened area increases due to protrusion of blackened part to the surrounding area of dot. In this respect dot concentration type is superior. However even in the case of dot concentration type, gradation and resolution are restricted by the matrix composition and hence it is important to have both types co-exist. To solve this problem, for example an IH method ("improved halftone") has been proposed. (Shashin Kogyo, Imaging Jan. 20, 1988, P35/Journal of Denshi Shashin Gakkai, Vol. 25 No. 1 P31, 1986.)
Next is explained hereinbelow the composition of threshold matrix in the display area corresponding to image signal.
FIG. 16A is the schematic diagram to show the composition of threshold matrix in the partial display area corresponding to one pixel of image signal and FIG. 16B is the schematic diagram to show 4 partial display areas corresponding to 4 pixels of image signal. Partial display area 40 is composed of 16 display pixels 31, forming 4.times.4 matrix and number of display pixels blackening one after another according to the order of the numbers described in the matrix, corresponding to the concentration of one pixel of image signal and the aggregate of blackened display pixels form the dot. FIGS. 17A to 17C are the drawings to show the state that blackened display pixels form the dot and FIG. 17A indicates the state of low concentration, FIG. 17B shows intermediate concentration and FIG. 17C shows the high concentration respectively of 4 pixels of image signal.
According to the aforesaid conventional method of image display, such problems exist that each dot displaying the low concentration pixel of image signal becomes extremely small and when said image display method is applied to offset printing, etc., uniform application of ink is not obtained or when it is applied to the plate or block copy, when the surface protective layer is peeled following exposure, the dot comes off together. When the high concentration (slightly lower than complete blackness) of image signal is displayed, ink is apt to stick to the gap between dots or the gap between dots is filled up due to inferior peeling of surface protective layer.
As stated above, especially at the preparation of the original plate of offset printing, it is known that problems exist as to the reproducibility of fine area.
With the electrophotographic laser beam printer, U.S. Pat. No. 4,800,442 (FIG. 7C) discloses the technology to define the minimum pulse width and maximum pulse width of laser but it makes no consideration of the preparation of original printing plate. Nothing is mentioned either as to the handling of input data of less than minimum pulse width or more than maximum pulse width and no measures to solve these problems are indicated.